Heat integration has become more widely used in the chemical process industries as energy costs have increased. However, until recently, the decision to invest capital in additional heat exchange capacity to save future energy costs remained solely a business and engineering judgement in which the operational constraints and incremental capital costs of heat integration were weighed against projected energy savings.
Designing two or more chemical process units with interdependent heating and cooling necessarily sacrifices some degree of operational flexibility. Thus one engineering objective in designing a heat integration scheme is to achieve the desired energy savings while minimizing the loss of flexibility.
More recently, however, environmental regulations have assumed a position of prominence in refinery design. Modifications to meet water quality standards and solid waste disposal guidelines add capital cost but generally do not require major modifications to existing refinery conversion processes. Improved wastewater treatment facilities and solid waste disposal techniques enable most conventional refineries to meet federal, state and local wastewater and solid waste regulatory standards.
Meeting air quality standards, however, poses a more challenging problem. These regulations limit stack effluent pollutant concentrations as well as pollutant mass flowrates. The more stringent regulations further limit the number of point sources as well as the total pollutant flow from the manufacturing facility. Examples of point sources in an oil refinery include process furnace stacks, steam boiler stacks and catalytic cracking unit regenerator flue gas stacks.
Turning now to refinery economics, the market demand for light C.sub.4 - olefins and C.sub.6 + aromatics as petrochemical feedstocks continues to grow. Typical oil refineries generate large quantities of paraffinic light gas which is burned as fuel or flared. Converting this light paraffinic gas to useful olefins and aromatics would transform an economic and environmental liability, i.e. excess light paraffinic gas, into saleable products. The resulting olefins are then easily converted to ethers which are useful for increasing gasoline octane. Thus, by upgrading light paraffinic gas to saleable gasoline, the gasoline market demand may be met with a lower rate of crude consumption.
Paraffin dehydrogenation and aromatization are strongly endothermic. Paraffin aromatization is believed to proceed via a two-step process, i.e. cracking or dehydrogenation followed by olefin aromatization. The olefin aromatization step is exothermic and mitigates the dehydrogenation endotherm to some extent; however, for a paraffin-rich feedstream, aromatization remains a net endothermic reaction.
Dehydrogenation of C.sub.2 -C.sub.10 paraffins requires a heat input of about 200 to 1200 BTU per pound of feed, more typically 400 to 700 BTU per pound of feed. The reaction temperature in the presence of ZSM-5 catalyst ranges from about 510.degree. C. to 705.degree. C. (950.degree. F. to 1300.degree. F.). Preheating the feed in a fired process furnace may partially crack the feed to form C.sub.2 - gas and coke. Paraffin dehydrogenation in a fluidized-bed reaction zone provides the additional option of transferring heat to the reaction zone by preheating the catalyst. Preheating the catalyst separately to around 870.degree. C. (1600.degree. F.) undesirably accelerates catalyst deactivation. The problem of transferring heat to the fluidized-bed process has clearly been an obstacle to its commercial development.
Maintaining and closely controlling relatively small pressure differentials, e.g. less than 5 psi, between the different reaction zones of a fluid catalytic cracking process is essential to its reliable operation. The catalyst regeneration section of a fluid catalytic cracking process operates at pressures up to about 450 kPa (50 psig), and the resulting regenerator flue gas must be depressured before it is exhausted to atmosphere. Orifice chambers typically comprising a plurality of perforate plates traversing a closed longitudinally extensive pressure vessel have gained wide acceptance in industry as a reliable means for depressuring regenerator flue gas and require only minor periodic maintenance to repair damage from catalyst erosion.
Flue gas flows out of the regenerator at temperatures in the range of about 590.degree. to 820.degree. C. (1100.degree. to 1500.degree. F.). In a conventional fluid catalytic cracking unit, this flue gas first flows through an orifice chamber which depressures the flue gas. The depressured flue gas then flows to a heat recovery unit, e.g., a steam generator, where the flue gas temperature falls to around 190.degree. C. (375.degree. F.). From the heat recovery unit, the cooled flue gas flows to a gas purification unit, e.g., an electrostatic precipitator, to remove catalyst fines, and is then exhausted to atmosphere through an elevated stack.